Aluminum-lithium filler alloy for brazing

ABSTRACT

An aluminum brazing alloy composite sheet is provided which can be utilized in both the vacuum brazing process and the controlled atmosphere brazing process for the brazing of aluminum parts. The brazing alloy composite sheet consists of an aluminum core alloy of the 3XXX, 5XXX or 6XXX type clad at least on one major surface with a lithium-containing aluminum filler alloy containing from about 0.01% to about 0.30% by weight lithium. The core alloy may contain up to about 2% by weight magnesium and is suitable for producing aluminum alloy heat exchanger assemblies, such as radiators or evaporators.

BACKGROUND OF THE INVENTION

Joining of aluminum by brazing is a well known process due to the strongand uniform joints that can be produced between aluminum parts ofvarying shapes and types. There are four major brazing processesutilized for the joining of aluminum parts, these are: (a) the flux dipbrazing process wherein the parts to be joined are dipped into a moltenflux bath utilizing a mixture of chloride and fluoride salts; (b) thefurnace brazing process which employs a small amount of flux, forexample a chloride salt; (c) the controlled atmosphere brazing processwhich uses a small amount of fluoridic salt and an inert gas atmosphere,for example nitrogen, argon or helium; and (d) the vacuum brazing methodwhich uses no flux but instead utilizes a reduced pressure atmospherefor the joining of the aluminum parts.

Each of these brazing methods has advantages and disadvantages. Forexample, the flux dip brazing process is associated with environmentalproblems arising out of the disposal of the used flux baths. Also, thealuminum parts joined by the flux dip brazing process must be thoroughlycleaned after fluxing to avoid the corrosive effects of the residualflux on the aluminum surfaces.

In the furnace brazing process, much less flux is utilized and the fluxis directly deposited on the surfaces of the parts to be joined. Thus,there is no flux bath disposal problem. The flux remaining on thesurface must be removed from the brazed surfaces to minimize thecorrosive effects of the flux. Nevertheless, the furnace brazing processcannot be readily utilized for the brazing of those aluminum alloyswhich have a relatively high magnesium content. Typical examples ofthose alloys which are not readily brazeable by the furnace brazingmethod are those aluminum alloys which belong to the AluminumAssociation 5XXX series.

The controlled atmosphere brazing process employs an inert gasatmosphere, for example argon or nitrogen gas atmosphere, in the brazingfurnace. The inert gas atmosphere brazing employs a relatively smallquantity of non-corrosive flux which need not to be cleaned from thebrazed surfaces. The fluoridic flux is expensive and in compositebrazing sheets undesirable interactions between the fluoride flux andmagnesium limit the maximum core alloy magnesium content to about 0.3%.

In vacuum brazing no flux is employed and the method is suitable forjoining those aluminum alloys which contain about 0.1-1.75% by weightmagnesium or even more. Due to the magnesium content of the aluminumalloy core, the brazed assembly is capable of exhibiting higher strengthproperties. Vacuum brazing requires a well sealed furnace, carefulcontrol of the pressure within the furnace, both of which may imparthigher costs to the brazing process. Additionally, in the vacuum brazingprocess, assembly tolerances must be critically controlled and thecleanliness of the parts is imperative.

For many applications, especially where strength was a majorconsideration, the use of aluminum alloys containing magnesium (Mg) upto about 2.00% was desired. Joining of such magnesium-containing alloysby brazing could only be accomplished through use of the vacuum brazingprocess. Vacuum brazing however, requires the installation of anexpensive vacuum brazing furnace and thus the process becomes capitalintensive.

Those aluminum alloys which are essentially Mg-free cannot be brazed bythe vacuum brazing process. Currently, for joining these Mg-freealuminum parts the controlled atmosphere brazing method, employing forexample nitrogen atmosphere, is used in the presence of a fluoridicflux. Where brazing of both Mg-free and Mg-containing aluminum alloyswas practiced, it was necessary to segregate the different types ofalloys and additionally, two different types of furnaces had to beinstalled, one for controlled atmosphere brazing and the other forvacuum brazing.

Thus, there has been a longstanding need for a filler alloy which couldbe utilized for the brazing of either magnesium-free ormagnesium-containing aluminum alloy parts by controlled atmospherebrazing or by vacuum brazing. Surprisingly, it has been found that analuminum filler alloy, containing a controlled quantity of lithium canbe readily employed for the brazing of Mg-free and Mg-containingaluminum alloys using either the controlled atmosphere (inert gas)brazing method or the vacuum brazing process. The aluminum filler alloyof the invention contains from about 0.01 to about 0.30 % by weight oflithium and as a major alloying element silicon, generally within thelimits from about 4 to about 18% by weight of the brazing alloy.

It has been recommended in U.S. Pat. No. 3,272,624 (Quaas) toincorporate 0.005-0.010% lithium into aluminum in order to obtain aself-fluxing filler alloy for welding aluminum parts together. The alloyis employed as an extruded or cast wire and is melted during the joiningprocess to obtain a self-fluxing, deoxidizing deposit in the joint area.If desired, up to 18.0% silicon can also be incorporated in the filleralloy. This alloy is employed as a substitute for fluxes containingchloride and fluoride salts since its residue does not need to beremoved from the produced joint. Recommended areas of applicationinclude carbon arc, oxy-acetylene and inert arc welding. There is norecognition that the presence of the lithium in the aluminum alloy wouldrender it suitable for use as a filler alloy for the brazing ofMg-containing aluminum parts in the presence of fluxes or as a filleralloy in the fluxless vacuum brazing of aluminum components.

U.S. Pat. No. 4,173,302 (Schultze et al) recommends the use of analuminum brazing alloy which contains 4-20% silicon and between 0.00001and 1.0% by weight, preferably between 0,005 and 0.1 by weight at leastone of the elements of sodium, potassium and lithium. According to thisreference the alloy can be utilized in the fluxless brazing ofaluminum-containing articles in a non-oxidizing atmosphere or in a lowvacuum. The addition of these alkali metals to the brazing alloy isclaimed to increase the corrosion-resistance of the brazed joint. Theuse of these alkali metal-containing brazing alloys is restricted tofluxless, controlled atmosphere brazing and the beneficial effects ofthese alkali metals are considered equivalent.

U.S. Pat. No. 5,069,980 (Namba et al) describes a clad aluminum alloysuitable for fluxless vacuum brazing. The cladding material is to beused on both sides of a core sheet. It contains 6-14% silicon, 0-0.6%magnesium, balance aluminum and additionally, at least one of thefollowing elements may also be incorporated in the cladding alloy forthe improvement of its corrosion-resistance: Pb, Sn, Ni, Cu, Zn, Be, Liand Ge. The role of these additives in the alloy are equated as far astheir corrosion-resistance improving effect is concerned.

It has surprisingly been discovered that the presence of lithium in thefiller alloy, when added in controlled amounts within the range fromabout 0.01 to about 0.30% by weight of the alloy, allows the use of thefiller alloy for brazing either by the controlled atmosphere brazingmethod or by the vacuum brazing process.

The universal applicability of the filler alloy of the invention for thebrazing of both magnesium-containing and magnesium-free aluminum alloyseliminates the need to segregate these alloys and further provides thefreedom to use whichever brazing method is preferred by the manufacturerof brazed aluminum assemblies.

BRIEF SUMMARY OF THE INVENTION

A filler alloy is provided which is suitable for brazing bothmagnesium-free and magnesium-containing aluminum alloys by using eitherthe controlled atmosphere brazing process or the vacuum brazing method.The filler alloy contains from about 0.01 to about 0.30% lithium (Li)and from about 4 to about 18% silicon (Si), zinc (Zn) up to about 2%,manganese (Mn) up to about 1%, impurities not exceeding about 0.15%,balance aluminum (Al). The filler alloy may also contain iron (Fe) in anamount not exceeding about 0.30%, copper (Cu) not exceeding about 0.10%.The filler alloy of the invention is useful for the brazing of aluminumcore alloys containing magnesium (Mg) up to about 1.30% by weight. Thepreferred filler alloy contains from about 0.01 to about 0.18% lithium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically shows the correlation between fillet area sizes andfiller alloy lithium contents within the range from 0% to 0.30% forvacuum brazed tube-to-header radiator assemblies using aluminum alloycores and lithium-containing filler alloy claddings.

FIG. 2 graphically shows the correlation between fillet area sizes andfiller alloy lithium contents within the range from 0% to 0.30% forcontrolled atmosphere brazed tube-to-header assemblies using aluminumalloy cores and lithium-containing filler alloy claddings.

FIG. 3 graphically compares the effects of Li, Ca, Na and Be in thefiller alloy on brazeability.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to the brazing of aluminum articles. Moreparticularly, this invention relates to a novel Li-containing aluminumfiller alloy suitable for the brazing of both Mg-free and Mg-containingaluminum alloy articles by either the controlled atmosphere brazingprocess or by the vacuum brazing method.

For the purposes of this invention, and as used hereinafter, the terms"controlled atmosphere brazing" or "CAB" refer to a brazing processwhich utilizes an inert atmosphere, for example nitrogen, argon orhelium in the brazing of aluminum alloy articles.

The term "vacuum brazing" as used herein refers to a process whichemploys reduced pressure in the brazing of aluminum alloy articles.

The filler alloy of the present invention, whether or not its use is inthe controlled atmosphere brazing process or in the vacuum brazingprocess, contains from about 0.01 to about 0.30% lithium. In addition tothe lithium content, the filler alloy also contains from about 4 toabout 18% Si. The filler alloy may also contain additional constituents,for example, zinc up to about 2%, manganese up to about 1%, iron inamounts up to about 0.30% and copper up to about 0.10%. The aluminumfiller alloy generally also contains the usual unavoidable impurities upto a total amount of about 0.15%. For vacuum brazing, the filler alloymay, if desired, also contain from about 0.1 to about 1.75% Mg.

When the filler alloy is employed in the vacuum brazing process, then itpreferably contains from about 0.01% to about 0.18% lithium. This amountof lithium in the filler alloy is sufficient to eliminate the need forgettering with magnesium and provides a more satisfactory atmosphere inthe vacuum brazing furnace. For the purposes of this invention,gettering refers to the removal of the oxygen content of the furnace bythe chemical reaction of the magnesium with the oxygen. Additionally,the elimination or the reduction of the required magnesium level in thevacuum brazing furnace through the use of the lithium-containing filleralloy extends the furnace life.

In the event the filler alloy is employed in the CAB process, forexample in combination with the well-known NOCOLOK™ fluoridic flux, thenthe lithium content of the filler alloy is also preferably maintainedwithin the range from about 0.01 to about 0.18%.

The novel filler alloy is generally employed in the form of a brazingsheet rolled from ingots having the desired alloy composition.Regardless of which brazing process the brazing sheet is going to beused in, for best results it is applied to the surface of the aluminumcore alloy through cladding. Cladding of the aluminum core alloy withthe brazing sheet is accomplished by methods well-known in the art, forexample by pressure welding through a rolling process. Depending on theassembly to be made the brazing or filler alloy sheet may be applied toone or both sides of the aluminum core alloy. The thickness of thebrazing sheet applied to the surface of the core alloy is usually withinthe range from about 5 to about 20% of the thickness of the aluminumcomposite. Thus, for example, if the thickness of the aluminum compositeis about 0.1 inch (2.54 mm), then the thickness of the cladding appliedto the surface of the aluminum core can vary between 0.005 and 0.020inch (0.127-0.508 mm).

The types of aluminum core alloys, which are clad with the novel filleror brazing alloy sheet, are generally selected on the basis of the enduse of the brazed assembly. Suitable aluminum core alloys which can beclad with the novel filler alloy composition include those aluminumalloys which are classified as 3XXX, 5XXX and 6XXX aluminum alloys bythe Aluminum Association, the 3XXX alloys being preferred.

The clad aluminum composite may be subjected to a heat-treatment toimprove its physical properties. Thus, the clad composites of thepresent invention may be subjected to a heat-treatment equivalent, forexample, to H-temper.

The clad aluminum alloy compositions of the present invention can bereadily employed for making brazed heat exchanger assemblies, such asradiators and components for such heat exchangers. Other applicationsare also possible, for example, utilization of the aluminum alloybrazing composition in the manufacture of evaporators.

The brazing of the assemblies made from the aluminum core alloys cladwith the Li-containing brazing sheet is accomplished according toprinciples well-known in the brazing art. For example, in the CABprocess, flux can be applied to the aluminum parts to be joined, thenthe assembly is preheated, for example to a temperature in the rangefrom about 425°-475° F. (224°-246° C.) The assembly is then transferredto a prebraze chamber where it is soaked for about 3-15 minutes at about750° F. (399° C.). Subsequently, the hot assembly is transferred to thebrazing furnace which is purged with dry nitrogen. The assembly is keptthen for 2-3 minutes at about 1095°-1130° F. (591°-610° C.) in the CABfurnace. The brazed assembly is then cooled, removed, and applied forits intended use.

If the vacuum brazing process is utilized for the joining of aluminumparts, no flux is applied to the joint area. The assembly to be brazedis usually preheated to about 425°-700° F. (224°-371° C.) and thenintroduced into the vacuum furnace. In the vacuum furnace, the preheatedassembly is heated in stages to about 1095°-1120° F. (591°-604° C.) andthen kept at temperature for about 3 minutes. Subsequently, the brazedassembly is cooled to about 1050°-1070° F. (566°-577° C.) and thenremoved from the vacuum furnace to be used for its intended purpose.

In the case of Mg-containing and also Mg-free aluminum core alloys,regardless of the brazing methods applied, the strengths of Jointsformed as measured by the area, weight or length of the filler in thejoints of the assemblies, are substantially the same. This factindicates that the novel, lithium-containing filler alloy can be readilyemployed for the production of vacuum or CAB brazed assemblies made fromboth Mg-free and Mg-containing aluminum core alloys when such corealloys clad with the novel Li-containing brazing sheet.

The following examples will further demonstrate the unique brazingcapability of the lithium-containing filler alloy and the applicabilityof such filler alloy for the brazing of both Mg-free and Mg-containingalloys using either the CAB method or the vacuum brazing process.

EXAMPLE 1

Experiments were conducted to establish the effectiveness of alithium-containing filler alloy for the production of satisfactorybrazed joints between Mg-containing aluminum core alloy parts. Theexperiments were conducted by brazing radiator test assemblies by boththe vacuum brazing and controlled atmosphere brazing methods.

Aluminum brazing sheets, having a thickness of 0.015 inch (0.0381 mm)and a composition shown in Table I, were roll clad on one side with thefiller alloys having varying lithium contents and an overall compositionshown in Table II. The cladding layer on the cores was equivalent toabout 10% of the total thickness of the clad composite. The compositeswere partially annealed to the H-24 temper in dry nitrogen at about 540°F. (282° C.) for a time period of about 4 hours. The partially annealedcore alloy-filler alloy composites were then used to make automotiveradiator tube and header test samples which were then brazed together bythe vacuum brazing method and also by the controlled atmosphere brazingmethod under conditions described below. The strength of the jointsformed in both types of brazing methods were then tested by measuringthe size of the areas of the fillets formed in the joints.

                  TABLE I                                                         ______________________________________                                        Composition of Aluminum Core Alloy                                            Element         Weight %                                                      ______________________________________                                        Si              0.10                                                          Fe              0.20                                                          Cu              0.33                                                          Mn              1.10                                                          Mg              0.45                                                          Ti              0.05                                                          Others (total)  0.15                                                          Balance aluminum                                                              ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Composition of the Li-containing Filler Alloy                                 Element         Weight %                                                      ______________________________________                                        Li               0.01-0.30*                                                   Si              9.50                                                          Fe              0.30                                                          Cu              0.10                                                          Mn              0.05                                                          Mg              0.05                                                          Zn              0.08                                                          Others (total)  0.15                                                          Balance aluminum                                                              ______________________________________                                         *The lithium contents of the filler alloys used vere varied within the        limits given in the Table.                                               

The tube and header test samples, each of which had a different lithiumcontent in the clad layer, were at first degreased and then assembled.The brazing of the assemblies proceeded as follows:

(a) Vacuum Brazing

The degreased assemblies were preheated in vacuum to 450° F. (232° C.)for 5 minutes, then they were transferred to the vacuum brazing furnacechamber where they were step-wise heated at first to 1000° F. (538° C.)in 10 minutes, then to 1095°-1120° F. (590°-604° C.) in 6 minutes. Theassemblies are then kept within the 1095°-1120° F. (590°-604° C.) rangefor about 3 minutes, then cooled. The strength of the brazed joints wasthen subjected to testing by determining the areas of the formedfillets. The results of tests are graphically shown in FIG. 1.

(b) Controlled Atmosphere Brazing

A NOCOLOK™ type flux, containing a fluoridic water-insoluble salt, wasdeposited on the surface of the degreased assemblies in an amountcorresponding to about 5 grams/m² surface. The fluxed assemblies werepreheated to 450° F. (232° C.) for 15 minutes then transferred to theprebraze chamber where they were soaked at 750° F. (399° C.) for 10minutes. Subsequently, the preheated assemblies were transferred to thebraze chamber (which was purged for 2 hours with dry N₂ prior to thebrazing) where they were kept at about 1100° F. (593° C.) for 3 minutesand then removed. The strengths of the brazed joints of the assemblieswere then tested by determining the areas of the fillets formed duringbrazing under controlled atmosphere. The test results are graphicallyillustrated in FIG. 2.

EXAMPLE 2

Test were also conducted to compare the effects of substituting lithiumwith calcium, sodium and beryllium in the filler alloy. Thus, sampleswere prepared by replacing the lithium content of the filler alloycomposition shown in Table II with substantially equivalent quantitiesof calcium, sodium and beryllium. For comparison, a lithium-containingsample was also used. The Li, Ca, Na and Be-containing filler alloyswere then tested by measuring the fillet lengths formed when subjectedto a conventional test based on the comparison of their respectivesurface tension which is a measure of brazeability. It was found thatthe fillet lengths and thus the strength properties of the filletsformed from the calcium, sodium and beryllium-containing filler alloyswere significantly below the strength level of the lithium-containingfiller alloy. The results of the comparative tests performed with Li,Ca, Na and Be-containing filler alloys are graphically depicted in FIG.3.

What is claimed is:
 1. A brazing sheet composite for joining aluminumparts by either a vacuum brazing process or the controlled atmospherebrazing process, which comprises:(a) an aluminum alloy core material;(b) a lithium-containing aluminum filler alloy cladding on the corematerial, the cladding being applied to at least one of the majorsurfaces of the core material, wherein the aluminum alloy core materialconsists essentially of silicon up to about 0.2%, magnesium up to about2%, manganese up to about 2%, iron up to about 0.3%, copper up to about0.40%, titanium up to about 0.1%, total impurities not exceeding about0.15%, balance aluminum; and wherein the cladding consists essentiallyof silicon within the range from about 4% to about 18%, lithium withinthe range from about 0.01% to about 0.30%, zinc up to about 2%,manganese up to about 1%, iron up to about 0.30%, copper up to about0.10%, magnesium not exceeding about 0.05%, impurities not exceeding atotal of 0.15%, balance aluminum.
 2. The brazing sheet composite ofclaim 1 wherein the lithium content of the filler alloy is maintainedwithin the range from about 0.01 to about 0.18%.
 3. The brazing sheetcomposite of claim 1 wherein the cladding is applied to one majorsurface of the core material and the thickness of the claddingcorresponds to from about 5% to about 20% of the total thickness of thecomposite.
 4. The brazing sheet composite of claim 1, wherein thecladding is applied to both major surfaces of the core material and thethickness of the cladding applied to each of the surfaces of the corematerial corresponds to from about 5% to about 20% of the totalthickness of the composite.
 5. The brazing sheet composite of claim 1wherein the core material is selected from the Aluminum Association3XXX, 5XXX and 6XXX series aluminum alloys.